1. Field of the Invention
The present invention relates to voltage doublers, and in particular relates to oscillating control signal generators thereof.
2. Description of the Related Art
A voltage doubler is commonly used to generate a voltage at a level higher than the supply voltage (not limited to twice of the supply voltage). In this manner, the generated voltage can be used in powering a device which has to be operated by a voltage higher than the supply voltage. For example, an LED driver, an LCD driver, a memory (e.g. DRAM, SRAM) and so on may require a voltage doubler.
In conventional techniques, a charge pump may be used to implement the voltage doubler. A charge pump may use a capacitor as a power storage element. According to an oscillating control signal, the charge pump charges the capacitor to provide a voltage at a level higher than the supply voltage. The charging of the capacitor depends on the frequency of the oscillation control signal. Generally, multiple oscillation signals, of different oscillation frequencies, are provided as candidates for the oscillating control signal. By a multiplexing design, the appropriate one of the oscillation signals is selected to input the charge pump to properly control the charging of the capacitor.
However, by the multiplexing design, it is difficult to timely switch the frequency of the oscillating control signal without affecting the efficiency of the voltage doubler. FIG. 1 shows a multiplexing design, which outputs an oscillating control signal fxc selected from the oscillation signals fx1 and fx2. The selection signal of the multiplexer Mux is named stb. In FIG. 2A, the frequency of the oscillating control signal fxc is switched from the frequency of fx1 to that of fx2 and, when the frequency switching occurs, the oscillation signal fx2 is low. As shown, the oscillating control signal fsc has an asymmetric duty cycle, which affects the efficiency of the voltage doubler. In FIG. 2B, the frequency of the oscillating control signal fxc is switched from the frequency of fx1 to that of fx2 and, when the frequency switching occurs, the oscillation signal fx2 rises to a high level. Accordingly, as shown, the oscillating control signal fsc cannot be timely transformed to the high oscillation frequency and loss of oscillation occurs. In FIG. 2C, the frequency of the oscillating control signal fxc is switched from the frequency of fx2 to that of fx1, and the switching of the selection signal stb is synchronized with the falling edge of the oscillating signal fx2. However, a glitch occurs due to PVT variations, which affects the efficiency of the voltage doubler.
Thus, an oscillating control signal generator generating an oscillating control signal working in a symmetric duty cycle and reacting timely and without glitches is called for.